Is there a more accurate way of knowing which vertice are affected by proportional editing?
This script / addon may help. It simply selects the vertices impacted by the proportional editing, starting from the current selection.

Once visualized, you can Ctrl+Z to come back to your current selection.



bl_info = {
    "name": "Select proportional editing impact",
    "author": "lemon acidjuice",
    "version": (1, 0),
    "blender": (2, 77, 1),
    "location": "View3D &gt; Select &gt; Edit mesh &gt; Select proportional editing",
    "description": "Selects vertices influenced by proportional editing",
    "warning": "",
    "wiki_url": "",
    "category": "3D View",
    }


import bpy
from bpy.types import Operator


def Influenced( u, selected, size2 ):
    for s in selected:
        if (s.co - u.co).length_squared &lt;= size2:
            return True
    return False

class EDIT_MESH_select_proportional(Operator):
    """Select proportional editing impact"""
    bl_idname = "editmesh.select_proportional"
    bl_label = "Select vertices influenced by proportional editing"
    bl_options = {'REGISTER', 'UNDO'}

    def execute(self, context):
        scene = context.scene
        obj = context.active_object

        size = scene.tool_settings.proportional_size        
        size2 = size * size

        mode = bpy.context.active_object.mode
        bpy.ops.object.mode_set(mode='OBJECT')

        selected = [v for v in obj.data.vertices if v.select]
        unselected = [v for v in obj.data.vertices if not v.select]

        for u in unselected:
            u.select = Influenced( u, selected, size2 )

        bpy.ops.object.mode_set(mode=mode)

        bpy.context.scene.update() 

        return {'FINISHED'}

# Registration

def add_select_menu(self, context):
    return self.layout.operator( EDIT_MESH_select_proportional.bl_idname )


def register():
    bpy.types.VIEW3D_MT_select_edit_mesh.prepend(add_select_menu)
    bpy.utils.register_class(EDIT_MESH_select_proportional)


def unregister():
    bpy.types.VIEW3D_MT_select_edit_mesh.remove(add_select_menu)
    bpy.utils.unregister_class(EDIT_MESH_select_proportional)

if __name__ == "__main__":
    register()